Test with go 1.7. Stop testing with 1.4 (#87)

diff --git a/.travis.yml b/.travis.yml
index 01a5a21..8e6ca75 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,9 @@
 language: go
 script: "./test.sh"
 go:
-    - 1.4
     - 1.5
     - 1.6
+    - 1.7
     - tip
 before_install:
   - go get github.com/axw/gocov/gocov